We visited Fort Sumter National Historical Park in Charleston, South Carolina.

The fort is on an island, so the only way to visit it is via a ferry operated by Fort Sumter Tours; best to book a few days ahead:

Aircraft carrier across the harbor, part of a museum:

Approaching Fort Sumter:

Entering the fort:

The first submarine in history to sink an enemy ship, in 1864:

Model of the fort:

Back to the ferry:

Fascinating.